The invention relates to improved aerofoil shapes for use as stator vanes or rotor blades in turbines of axial flow turbomachines, such as gas turbine engines.
Turbomachines are used to add energy to a working fluid and/or to extract energy from it. Accordingly, they may comprise compressors and/of turbines. For example, gas turbine engines typically comprise three main sections; a compressor section, a combustion section and a turbine section. Air from the atmosphere is drawn into and is compressed by the compressor. It is then passed into the combustion section where fuel is added and the mixture ignited so that an energised working fluid is created in the form of a pressurised hot gas. The working fluid passes from the combustion section to the turbine section where its energy is extracted by turbine blades and used to turn the compressor via a turbine shaft and do additional work. Eventually the working gas, now at much reduced temperature and pressure, is discharged to atmosphere via an exhaust duct system.
In the present invention, the means used to convert turbine working fluid energy into shaft rotational energy is a system of aerofoils comprising axial flow rotor blades and stator vanes. The rotor blades and stator vanes are arranged to intercept the working fluid as a number of axially successive annular rows. Each rotor blade is attached to a turbine rotor disc or drum via a blade root portion, the disc or drum being mounted on a rotor shaft, the longitudinal centre line of which defines the rotational axis of the turbine. The stator vanes are fixed, e.g., to a circumscribing turbine casing or to an inner static drum, and rows of vanes and blades alternate with each other so that each row of blades is paired with a preceding row of stator vanes. Each such pair of rows is collectively termed a stage and a turbine will comprise at least one stage.
Whereas the function of the rotor blade rows is to extract energy from the working fluid and transfer it to a turbine rotor disc or drum and hence to the shaft, the function of the stator vanes is to smooth the flow of the working fluid and then direct it at an optimum outlet angle to the rotor blades so that efficient energy transfer may be achieved there to turn the rotor. The efficiency with which both blades and vanes perform their function is of vital importance in determining stage efficiency.
In the gas turbine engine field, aerofoils of turbine vanes and blades have respective generic types of cross-section profile and may bear a strong visual likeness one to another, notwithstanding scale differences usually dependent upon engine size. However, on inspection it is found there are measurable differences of aerofoil profiles not only between engines of different make and type but also between turbine stages of the same engine. Further, such differences way have significant effects on turbine efficiency. Similarly, there are differences in other aspects of turbine stage design which alone or in combination also have an effect. Small differences in such design features, which may appear minimal or unimportant to those unskilled in the art, may in fact have a significant effect on turbine stage performance.
Hence, vane and blade geometrical shapes, their positional relationships to each other and also to the stream of working fluid have an effect on turbine efficiency and thus on turbomachine efficiency overall. In known state-of-the art gas turbine engines, the turbine stage efficiency is currently in the region of 90% and at such high efficiency it is regarded as now very difficult to improve by even parts of 1%. Nonetheless, it is an object of the present invention to increase turbine stage efficiency by a significant amount.
In part, the present invention incorporates and improves upon previous teachings in respect of so called xe2x80x9cControlled Flowxe2x80x9d principles by the present inventor and others. In particular, see patent GB 2 295 860 B xe2x80x9cTurbine Bladexe2x80x9d, directed particularly at steam turbines. Other patents showing similar principles include U.S. Pat. No. 5,326,221 Amyot et al. (for steam turbines) and U.S. Pat. No. 4,741,667 Price et al. (for gas turbines).
Definitions
For the purposes of the present invention, it will be understood that the term xe2x80x9cvanexe2x80x9d refers to the stator blades which precede the rotor blades in turbomachines, including the so-called xe2x80x9cnozzle guide vanesxe2x80x9d in gas turbine engines, which function to direct the hot gases from the combustor onto the first stage of turbine rotor blades. Also, when the word xe2x80x9cbladexe2x80x9d is used without the qualifying words xe2x80x9cstatorxe2x80x9d or xe2x80x9crotorxe2x80x9d, it should be taken to mean xe2x80x9crotor bladexe2x80x9d
The radially innermost extremity of the aerofoil portions of axial flow blades and vanes will be termed their xe2x80x9cplatform regionxe2x80x9d (even though the radially innermost portion of a gas turbine rotor blade is usually termed a xe2x80x9crootxe2x80x9d), and the radially outermost extremities of their aerofoil portions will be termed their xe2x80x9ctip regionxe2x80x9d (despite the fact that blades and vanes can have radially outer shrouds).
The xe2x80x9cpressurexe2x80x9d surface of an aerofoil section shape is its concave side and the xe2x80x9csuctionxe2x80x9d surface is its convex side.
A xe2x80x9cprismaticxe2x80x9d aerofoil is designed such that the notional aerofoil sections of the blade or vane, each considered orthogonal to a radial line from the turbine axis, have the same shape from the aerofoil platform region to the aerofoil tip region, are not skewed, i.e., have the same setting angle from the platform region to the tip region, and are xe2x80x9cstackedxe2x80x9d one on top of another so that their leading edges and their trailing edges collectively form straight lines in the radial direction.
The outlet angle xcex1 of an aerofoil is the angle, relative to the circumferential direction of the rotor, that the working fluid leaves a vane or blade row and is derived from the relationship:
xcex1=sinxe2x88x921(T/P), 
where T is the throat dimension and P is the pitch dimension.
Throat dimension T is defined as the shortest line extending from one aerofoil trailing edge normal to the suction surface of the adjacent aerofoil in the same row, whereas pitch dimension P is the circumferential distance from one aerofoil trailing edge to the adjacent aerofoil trailing edge in the same row at a specified radial distance from the platform region of the aerofoil.
The setting angle xcex2 is the angle through which any particular aerofoil section at a station along the height or span of the aerofoil is displaced in its own plane from a predetermined zero datum. The datum may, for example, be taken as being where the aerofoil section has the same xe2x80x9cstagger anglexe2x80x9d, i.e. the same orientation relative to the turbine axis, as a known prismatic aerofoil in a known turbine utilising such aerofoils.
The xe2x80x9cchord linexe2x80x9d is the shortest line tangent to leading and trailing edge radii of an aerofoil section. The xe2x80x9cchord lengthxe2x80x9d is the distance between two lines normal to the chord line and passing through the points where the chord line touches the leading and trailing edges respectively.
The xe2x80x9caxial widthsxe2x80x9d of an aerofoil is the axial distance between its leading and trailing edges, i.e., the distance between its leading and trailing edges as measured along the rotational axis of the turbine.
According to a first aspect of the present invention, a turbine stator vane is for use in a ring of similar vanes arranged in an axial flow turbine having an annular path for a turbine working fluid, the vane comprising an aerofoil spanning the annular path and having a radially inner platform region, a radially outer tip region, an axially forward leading edge and an axially rearward trailing edge, the aerofoil having a pressure surface and a suction surface which are respectively convex and concave between the platform region and the tip region in a plane extending both radially of the annular path and transversely of the axial direction, the trailing edge of the aerofoil being straight from the platform region to the tip region and oriented radially of the annular path, and said convex and concave curvatures of the aerofoil pressure and suction surfaces being achieved by rotational displacement of the aerofoil sections about the straight trailing edge, the axial width of the aerofoil being substantially constant over substantially all of the aerofoil radial height and the chord line at mid-height aerofoil sections being shorter than the chord lines in aerofoil sections at platform or tip regions.
In the context of a gas turbine engine, the invention in its first aspect may be applied to the aerofoils of nozzle guide vanes in the first or high pressure stage of the turbine, but also to the stator vanes of succeeding stages. Because the chord line at mid-height aerofoil sections is shorter than the chord lines in aerofoil sections at both the platform regions and the tip regions, the aerofoil exhibits a so-called xe2x80x9ccompound leanxe2x80x9d appearance when viewed on its leading edge, in which the aerofoil is skewed in the same circumferential direction at both radial extremities.
In accordance with a second aspect of the invention, if the aerofoil is that of a nozzle guide vane at the entry to a gas turbine, the aerofoil is preferably positioned in relation to the axial length of the turbine such that the trailing edge of the aerofoil is in a divergent part of the gas flow passage, whereby the trailing edge of the aerofoil is substantially longer than its leading edge.
In the case of a nozzle guide vane aerofoil, the aerofoil""s platform and tip outlet angles are preferably of substantially the same value, for example, not more than about 10 degrees, preferably in the range 8-10 degrees. The aerofoil""s outlet angle at mid-height of the aerofoil may be in the range 13-16 degrees, preferably approximately 14 degrees.
Conveniently, the aerofoil is of approximately constant aerofoil cross-section from its platform region to its tip region.
In accordance with a further aspect of the invention, a turbine stage comprises a row of stator vanes as described above, and a row of rotor blades in flow sequence with the vanes, in which the blades comprise aerofoils having a radially inner platform region, a radially outer tip region, an axially forward leading edge and an axially rearward trailing edge, each blade aerofoil having a pressure surface and a suction surface which are respectively convex and concave between the platform region and the tip region in a plane extending both radially of the annular path and transversely of the axial direction, said convex and concave curvatures of the aerofoil pressure and suction surfaces being achieved by rotational displacement of the aerofoil sections about a radial line through the aerofoil, each aerofoil having outlet angles which are smaller near its platform and tip regions than at mid-height.
From an aerodynamic point of view, each blade aerofoil ideally has a radially oriented straight trailing edge, the rotational displacement of the aerofoil sections being about the straight trailing edge, though this ideal may be compromised by the dynamic design requirements of the blades.
To reduce dynamic loading at the root fixings and the platform, the blade aerofoil may taper from its platform region to its tip region, such that its chord length reduces over the blade aerofoil""s radial height from a maximum at its platform region to a minimum at its tip region and its leading edge has a backward lean in the axial direction.
In yet another aspect, the invention provides a turbine stage comprising a row of nozzle guide vanes having aerofoils as described above, and a row of rotor blades in flow sequence with the vanes, in which the blade aerofoil platform and tip outlet angles are in the range 14-17 degrees, preferably about 16 degrees. The blade aerofoil outlet angle at mid-height of the aerofoil may be in the range 18-21 degrees, preferably about 19 degrees
The invention is believed applicable whether the aerofoils are shrouded or unshrouded, i.e., whether the aerofoils are joined to a structure forming an outer wall of the passages between adjacent aerofoils, or are not so joined, but are free at their radially outer or tip regions.
Further aspects of the invention will be apparent from the following description and claims.